In all current plasma spray systems using powder injection, the apparatus is such that the arc column itself or its ionized plume is used as the extremely high temperature heat source. This fact is of extreme importance in applicant's pending U.S. patent application Ser. No. 024,485, filed Mar. 11, 1987, by forcing the arc column to extend much further beyond the nozzle exit than in conventional plasma torches. In accordance with FIG. 1 of the drawings, a conventional plasma spray torch 10' is illustrated, in which the water cooling means have been purposely eliminated for simplicity purposes from that figure. An electrically insulating body piece 10 of cylindrical, cup-shaped form supports a cathode electrode 12 coaxially and projecting towards but spaced from a second body piece 11 closing off the open end of the cup-shaped form body piece 10, at the end opposite that supporting the cathode electrode 12. The second body piece 11 is provided with an axial bore 11a constituting the plasma spray torch nozzle passage 9. An arc 17 is formed by connecting an electrical potential difference across the cathode electrode 12 and the second body piece 11, acting as the anode. The arc 17 passes from the electrode 12 to the inner wall of the nozzle passage 9. Its length is extended by a flow of plasma forming gas as shown by the arrow G which enters the annular manifold 24 about the cathode electrode 12 through a gas supply tube 15. Tube 15 connects to the body piece, and through an aligned radial hole 15a within the side of that cylindrical body piece. A transverse partition 13 of insulating material, like that of body piece 10, supports the electrode 12. The partition 13 is provided with a number of small diameter passages 23 leading into the nozzle passage 9 with flow about the tapered tip end 12a of the cathode electrode 12. Powder to be sprayed, as indicated by the arrow P, passes into the arc-heated gases at a pint beyond the anode foot 18 of arc 17. Powder is introduced through the tube 16 and flows into a passage 16' aligned therewith and opening to the bore 11a in such a manner as to assure centering of the powder flow as best possible, along the hot gas jet 25 which exits from the end of nozzle 9.
An extremely bright conical arc region 19 extends a short distance beyond the exit of the nozzle 9, with this region constituting the further extension of the ionized gas species. Tremendous heat transfer rates occur within the conical region 19. As may be appreciated, there is added gaseous heating of particle P flow beyond the ionized zone 19 within the hot gas jet 25. Further, the particles pick up speed in the high velocity (but subsonic) jet 25 to strike the surface of the workpiece 22 and to form the coating 21 on the surface of the workpiece. Exemplary, the conventional plasma spray torch 10' is provided with a flow of 100 SCFH of nitrogen gas G using a nozzle passage 9 bore diameter of 5/16-inch, and the torch is provided with an operating current of 750 amp and an arc voltage of 80 volts. The ionized zone or region 19 is observed to extend about 1/3-inch beyond the end 9a of the nozzle. The gross power level reached is 60 Kw. The combined cathode and anode losses are about 30 volts with a net heating capability (I.sup.2 R heating of the gas) of 37.5 Kw. Assuming an additional heat loss to the cooling water of 20%, the gas heating amounts to 30 Kw. The enthalpy increase of the plasma gas in such conventional system under the conventional operating parameters set forth above is about 14,500 Btu per pound.
In all current plasma equipment employing so-called low-voltage arcs (around 80 volts) the apparatus operates as shown in FIG. 1. Where the material to be sprayed is heat-insensitive, the high heating zone is of great benefit. However, for material which can be heat-damaged, such plasma systems have never been able to match the quality of the "D-GUN" or my prior high-velocity combustion system as set forth in U.S Pat. No. 4,416,421.
Prior plasma torches have relied on almost instantaneous particle heating as the powder passes into and through cone 19 of FIG. 1. Many of these particles (particularly smaller sizes) actually become fully molten, and perhaps even vaporized. A heat-sensitive material such as tungsten carbide (WC) decarbonizes to form W.sub.2 C which may not be desirable. In addition, the molten particles may become heavily oxidized. The "D-GUN" and apparatus of U.S. Pat. No. 4,416,421 provide an extended high-velocity heat source of much reduced temperature compared to the nearly instantaneous heating of conventional plasma equipment. The entrained powder particles in such apparatus are heat-softened rather than being melted, thus retaining their chemical composition and becoming only lightly oxidized even when sprayed on to a workpiece held in the open atmosphere.
FIG. 2 is a longitudinal sectional view of an improved, non-transferred plasma arc torch having an extended arc in accordance with the principals of my copending parent U.S. application Ser. No. 024,485. FIG. 2a is an enlarged, longitudinal sectional view of the exit end of nozzle bore 31a of the plasma-arc torch of FIG. 2. Referring to FIGS. 2 and 2a, the improved plasma spray torch is indicated generally at 10 and employs a cylindrical, electrically insulating body piece 30 similar to that at 10' in the prior art plasma torch of FIG. 1. Body piece 30 is closed off by a second cylindrical body piece 31 and the opposite end of the body piece 10 includes a transverse end wall 30a supporting coaxially and projecting through annular chamber 41 internally of the body piece 30, a cathode electrode 32. The foot 32a of the cathode electrode 32 projects into a conical reducing section 35 of bore 31a defining a torch nozzle passage 34. A high vortex strength plasma gas flow creates an extended ionized arc column zone achieved by having a gas supply pipe or tube 26 tangentially disposed with respect to the annular chamber 41 surrounding the cathode electrode 32, with the gas flow as shown by arrow G entering chamber 41 tangentially as clearly seen in FIG. 2b through passage 33 and exiting through the conical reducing section 35 leading to bore 31a. As such, the conical reducing section 35 smoothly passes the vortex flow into the reduced diameter nozzle passage 34. The principle of conservation of angular momentum creates a greater vortex strength with reduction of the outer boundary diameter of the gas flow. A small diameter core of the vortex exhibits low gas pressure relative to that of the gas layers near the passage 34 wall (bore 31a). An extended arc column 37 results with that arc column positioned to pass through the low pressure core and well beyond the exit 34a of nozzle 34. By physical phenomena, not well understood by the applicant, a reduction of the nozzle 34 diameter and/or an increase in arc current creates a greater than critical pressure drop in its passage through the nozzle 34 to the atmosphere to eliminate the vagaries of the arc anode spot associated with the subsonic counterpart. With supersonic flow, the anode region becomes more diffused and spreads over the inner wall of nozzle 34 near the nozzle exit 34a and over a thin circumferential radial region of the body piece 31 surrounding the exit 34a of the nozzle. The extended arc 37 (ionized zone) is of reduced diameter compared to the ionized zone 19 of the prior art torch, FIG. 1. Its length extending beyond the nozzle exit 34a is also significantly increased over the length of the ionized zone 19 of the prior art device, FIG. 1. The torch 10 of FIGS. 2, 2a, for example, operates adequately using 120 SCFH of nitrogen under an applied voltage of 200 volts across the gap between the cathode electrode 32 and the anode 31 at a current of 400 amp. In such example, the nozzle diameter was 3/16-inch and under operating parameters, the ionized zone extends 11/4 inches beyond the nozzle exit 34a, with the electrode losses again about 30 volts, the net gas enthalpy (after the 20% cooling loss) reach 27,000 Btu per pound; nearly double that of the prior art apparatus of FIG. 1.
FIG. 2a illustrates, in an enlarged view, the extended arc 42 with its anode foot 36 at the exit of nozzle 34, and with the cavity 39 eroded into nozzle 31 by the co-action of the intense anode heating within the presence of atmospheric oxygen which is readily available. The formation of cavity 39 takes several hours of operation, and as it erodes deeper into the nozzle, the erosion rates become less. This lessening is probably due to exiting gas inhibiting oxygen flow into cavity. In any event, the cavity is unsightly and is best eliminated.
It is therefore a present object of the present invention to provide a method and apparatus for the extension of the life of circumferential anode region at the end of the exit nozzle of plasma torches of the type set forth in copending U.S. application Ser. No. 024,485.